Fuel cells have been used as a power source in many applications. For example, fuel cells have been proposed for use in electrical vehicular power plants to replace internal combustion engines. In proton exchange membrane (PEM) type fuel cells, hydrogen (or hydrogen containing gas) is supplied to the anode of the fuel cell and oxygen is supplied as the oxidant to the cathode. The oxygen can be either a pure form (O2) or air (a mixture of O2 and N2). PEM fuel cells include a membrane electrode assembly (MEA) comprising a thin, proton transmissive, non-electrically conductive, solid polymer electrolyte membrane having the anode catalyst on one face and the cathode catalyst on the opposite face.
The term “fuel cell” is typically used to refer to either a single cell or a plurality of cells (stack) depending on the context. A plurality of individual cells are typically bundled together to form a fuel cell stack and are commonly arranged in electrical series. Each cell within the stack includes the membrane electrode assembly (MEA) described earlier, and each such MEA provides its increment of voltage. A group of adjacent cells within the stack is referred to as a cluster. By way of example, some typical arrangements for multiple cells in a stack are shown and described in U.S. Pat. No. 5,763,113.
The fuel cell stack receives a cathode input gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack.
The fuel cell stack includes a series of flow field plates or bipolar plates positioned between the several MEAs in the stack. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided in the anode side of the bipolar plates that allow the anode gas to flow to the anode side of each MEA. Cathode gas flow channels are provided in the cathode side of the bipolar plates that allow the cathode gas to flow to the cathode side of each MEA. The bipolar plates are made of a conductive material, such as stainless steel, so that they conduct the electricity generated by the fuel cells from one cell to the next cell as well as out of the stack.
It has previously been proposed by the inventors in U.S. Patent Application 2005/0058864 A1, titled “Nested Bipolar Plate for Fuel Cell and Method”, published Mar. 17, 2005, the entire disclosure of which is hereby incorporated herein by reference, that the thickness or repeat distance of a fuel cell stack can be reduced by nesting the flow channels in the active feed region of the fuel cells. In this design, the fuel cell stack includes two MEAs for adjacent fuel cells in the stack (there is one MEA per bipolar plate). Each MEA includes a membrane of the type referred to above, an anode side catalyst layer and a cathode side catalyst layer. An anode side gas diffusion media layer is positioned adjacent to the MEA and a cathode side gas diffusion media layer is positioned adjacent to the MEA. A bipolar plate assembly is positioned between the diffusion media layers. The bipolar plate assembly includes two stamped metal bipolar plates that are assembled together in the nested configuration. The nested plates define parallel anode gas flow channels and parallel cathode gas flow channels, where the anode flow channels provide a hydrogen flow to the anode side of the MEA and the cathode flow channels provide airflow to the cathode side of the MEA. Additionally, the plates define coolant flow channels through which a cooling fluid flows to cool the fuel cell stack.
A fuel cell in a fuel cell stack that provides a transition from nested bipolar plates in the active feed region of the stack to non-nested bipolar plates in the inactive feed regions of the stack without giving up the reduced stack thickness provided by the nested plates or changing the size of the flow channels has previously been proposed by the inventors in U.S. patent application Ser. No. 11/009,378, titled “Reactant Feed for Nested Stamped Plates for a Compact Fuel Cell”, filed Dec. 10, 2004, the entire disclosure of which is hereby incorporated herein by reference. Particularly, the diffusion media layers in the fuel cells of the stack are removed in the inactive feed regions where the bipolar plates are non-nested so that the volume necessary to maintain the size of the flow channels is provided without the need to increase the distance between adjacent MEAs. Additionally, the membrane of the MEAs would not be catalyzed in the inactive feed regions. A thin shim can be provided between the membrane and the plates in the inactive feed regions to support the membrane where the diffusion media layer has been removed to prevent the membrane from intruding into the flow channels and blocking the reactive flow. However, clearance gaps in the inactive feed regions are required to ensure adequate contact of diffusion media and bipolar plates in the active feed regions to reduce the electrical contact resistance. These clearance gaps result in variations in reactant flow uniformity and pressures within a cell, and from cell to cell within a stack, due to variations in tolerances in part thicknesses.
Accordingly, what is needed in the art is a method to control clearance gaps in the inactive feed regions to provide reactant flow uniformity and pressure within fuel cells and fuel cell stacks utilizing nested bipolar plates in the active feed regions.